The present invention relates to air data systems that provide accurate compensation of sideslip of an air vehicle utilizing independent probes that are not pneumatically coupled, but which have processors for interchanging electrical signals between the probes. These probes are sometimes referred to as multi-function probes (MFPs). One type of MFP is the SmartProbe(trademark) sold by B. F. Goodrich Company. Multi-function probes include processing circuitry located at the probe itself as part of its instrument package. During sideslip of the air vehicle, compensation of various local (to the probes) parameters or signals, such as angle of attack and static pressure, is necessary for accurate determination of aircraft angle of attack and other aircraft parameters including determination of altitude from static pressure or other means. This requirement for accuracy in altitude indications is particularly important in Reduced Vertical Separation Minimum (RVSM) space areas of the air traffic control system.
In conventional air data systems, probes on opposite sides of an aircraft can be pneumatically connected so that the pressure signals are averaged between the right side of the aircraft and the left side of the aircraft to provide a static pressure that is xe2x80x9cnearly truexe2x80x9d. In most conventional systems, although corrections are made for Mach number and aircraft angle of attack, it is rare that neglecting sideslip effect will introduce enough error to warrant a correction based on sideslip for the cross coupled probes.
However, MFPs are connected only electrically in order to eliminate the need for pneumatic tubing passing between the opposite sides of the aircraft or between probes on the same side of the aircraft. This means that each probe is pneumatically independent, even if it is electrically communicating with other probes. In the RVSM space, there is a need for dual redundant systems for static pressure estimation. While information can easily be exchanged between the processing circuitry of different probes, the need for determining sideslip effects remains. Computational fluid dynamic analysis has shown that position errors can be up to 600 feet per degree of sideslip under typical RVSM flight conditions at, for example, 41,000 feet and a Mach number of 0.8. It is thus apparent that the sideslip effect must be corrected to obtain the necessary accuracy for certification by aviation authorities.
The present invention relates to multi-function air data sensing systems which provide for redundancy in correcting for sideslip of an aircraft arriving at various air data parameters, such as aircraft angle of attack, pressure altitude, and Mach number. Aerodynamic sideslip is a measure of the magnitude of a cross component of airspeed to the forward component of airspeed. Compensation information exchanged between MFPs, such as a differential and local angle of attack between the two sides of an aircraft, can provide an indication of sideslip effect utilizing the system disclosed herein. Using values of local angle of attack, for example at two separate MFPs, provides information that corresponds to aircraft parameters or variables of angle of attack and angle of sideslip.
A predictor-corrector method can be used to iteratively calculate aircraft parameters based on assumed free stream variables. As an example, knowing the local angle of attack at a single probe, a prediction is made for the aircraft angle of attack based on an assumed value of aircraft angle of sideslip. This is done for a second probe on the same aircraft. A comparison is made between the two predicted values of aircraft angle of attack. If they differ within a selected tolerance, it is deduced that the assumed aircraft angle of attack and aircraft angle of sideslip are correct for that combination of local angle of attack measurements at the two MFPs. If the difference between the two predicted aircraft angles of attack is not within a specified tolerance, it is assumed that the aircraft angle of attack is actually the average of the two predictions. Predictions for aircraft angle of sideslip are then made, with each prediction being made using the local angle of attack from a different one of the two probes and the new assumed aircraft angle of attack. A comparison is then made between the two predicted aircraft angle of sideslip values. If the two predicted aircraft angle of sideslip values are within a predetermined tolerance range of each other, then the iterative process is completed and the aircraft parameters of angle of sideslip and angle of attack are determined based upon the predictions and assumptions. If the two predicted aircraft angle of sideslip values are not within tolerance, the process continues.